1. The Field of the Invention
The present invention generally relates to rotating machinery. In particular, some example embodiments relate to an x-ray tube bearing assembly with a resonant frequency tuned to enable operation at one or more desired operating frequencies.
2. The Related Technology
X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both medical and industrial. For example, such equipment is commonly employed in areas such as medical diagnostic examination and therapeutic radiology, semiconductor manufacture and fabrication, and materials analysis.
Regardless of the applications in which they are employed, x-ray devices operate in similar fashion. In general, x-rays are produced when electrons are emitted, accelerated, and then impinged upon a material of a particular composition. This process typically takes place within an evacuated enclosure of an x-ray tube. Disposed within the evacuated enclosure is a cathode, or electron source, and an anode oriented to receive electrons emitted by the cathode. The anode can be stationary within the tube, or can be in the form of a rotating annular disk that is mounted to a rotor shaft which, in turn, is rotatably supported by a bearing assembly. The evacuated enclosure is typically contained within an outer housing, which also serves as a reservoir for a cooling fluid, such as dielectric oil, that serves both to cool the x-ray tube and to provide electrical isolation between the tube and the outer housing.
In operation, an electric current is supplied to a filament portion of the cathode, which causes a cloud of electrons to be emitted via a process known as thermionic emission. A high voltage potential is placed between the cathode and anode to cause the cloud of electrons to form a stream and accelerate toward a focal spot disposed on a target surface of the anode. Upon striking the target surface, some of the kinetic energy of the electrons is released in the form of electromagnetic radiation of very high frequency, i.e., x-rays. The specific frequency of the x-rays produced depends in large part on the type of material used to form the anode target surface. Target surface materials with high atomic numbers (“Z numbers”) are typically employed. The target surface of the anode is oriented so that the x-rays are emitted as a beam through windows defined in the evacuated enclosure and the outer housing. The emitted x-ray beam is then directed toward an x-ray subject, such as a medical patient, so as to produce an x-ray image.
In x-ray devices that include a rotating anode, the intensity of the emitted x-ray beam depends in part on the rotational frequency of the anode, usually expressed in Hertz (“Hz”). To obtain high x-ray beam intensities required for certain applications, such as in high-speed CT scanners, the rotating anode may be required to operate at frequencies as high as 150 Hz or higher, for instance.
Regardless of the actual or desired operating frequency, all rotating anode designs are characterized by one or more resonant frequencies. Vibrations of the rotating anode caused by imbalances in the anode or other rotating components reaches a maximum when the anode is operated at or near a characteristic resonant frequency. Although rotating anodes may briefly rotate at a resonant frequency during acceleration to an operating frequency above or below the resonant frequency, maximized vibration levels at the resonant frequency prevent prolonged operation at the resonant frequency.
In the case of conventional x-ray devices, the characteristic resonant frequency of a rotating anode is measured after manufacture of the rotating anode and bearing assembly has been completed. Once the resonant frequency has been determined, the manufacturer typically specifies one or more permitted operating frequencies. A user is thus constrained to operate at the operating frequencies specified by the manufacturer without regard to the operating frequencies that may be desired by the user to achieve a particular x-ray beam intensity.
The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one exemplary technology area where some embodiments described herein may be practiced.